Metal piping is an essential part of the infrastructure of every city. It is used in a wide variety of circumstances, from the most benign such as the interior of a home to the most extreme, such as acidic soil which freezes and thaws each year. Piping, of course, can be expensive and difficult to replace, particularly if it must be replaced after failure. Thus any method of improving the time in service of piping can result in significant savings to the pipeline user.
At present the pipeline industry is searching for higher performance coatings which can be effectively applied to prevent damage to pipes from corrosion and external mechanical stress. The presently prevalent commercial products are bituminous tape materials. These are essentially a layer of asphalt-like material with an adhesive backing. Although the existing bituminous tape materials are cheap, they have low impact resistance and their application is laborious and costly. Essentially, the tape has to be wound around the pipe. Certain pipeline industry manufacturers would like to phase out bituminous materials and replace them with thin, flexible, light, high performance coatings. In this invention we have developed a two-component polyurethane composition which can be effectively applied by spray and/or curtain coating onto metal pipes to obtain such a coating. This material is tough, has the right amount of flexibility and has excellent impact properties and with minimum cathodic disbonding.
An ideal high performance coating for metal pipe would have the following properties:
Ease of Handling--The material should be one that can be applied by both spray and curtain coating methods. Once applied, it should not come off easily or add significantly to the weight, size, or handling properties of the pipe. Also, the coating should not drip and should dry or cure quickly.
Durability--The material should adhere strongly to the metal substrate. In this case the inventors are particularly interested in coating ductile cast iron. Ductile cast iron is extremely hard, but is very vulnerable to corrosion, and coating will help, but corrosion will occur at any place where the coating is flawed. The coating should have high impact resistance, so that the coating does not chip off during transport and installation, and also maintain some degree of flexibility as reflected in its tensile strength, tear strength, and elongation. These traits accommodate the pipe's inevitable response to temperature changes and long-term mechanical stress.
Water Resistance--One of the major functions of the coating is to keep water away from the metal surface. The coating itself must be highly resistant to liquid water over time. Also, it is desirable that the coating be largely impenetrable to water vapor, i.e., have a low vapor transmission rate. Resistance to liquid water and resistance to water vapor do not necessarily go hand in hand. In the case of coatings for oxidizable metals, it is particularly important to keep water vapor away from the metal, as small pockets of rust can form, which will force the coating to "bubble up" and the pipe to corrode. Finally, the coating must exhibit a minimum of cathodic disbonding, i.e., delamination of the coating at the site of a coating defect.
Environmentally Benign--Another significant advantage would be if the formulation is solventless, i.e., does not use any of the volatile organic compounds which will eventually be banned due to environmental concerns.
The inventors have found that a variation of the type of coatings previously applied to concrete is especially useful because it encompasses all the listed advantages. Such concrete coatings are disclosed in U.S. Pat. No. 4,877,829, issued to Vu et al. Oct. 31, 1990, the text of which is incorporated herein by reference as if set forth in full.